Firefly and PC GAMESS-related discussion club

Firefly's documentation has been updated and can be obtained from the "Manuals" section. Changes are as follows:

- added descriptions for various keywords new to Firefly 8.1.1;- completed keyword descriptions for the $MASS, $DATA, $P2P, $LP2P, and $DRC groups;- added text on the memory limitations of Firefly;- added a release history for documentation;- improved layout;- various minor improvements.